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The Term Brown Dwarf (BD) Refers to Objects with Masses Below the Limit for Stable Hydrogen Fusion in Stellar Interiors

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The Term Brown Dwarf (BD) Refers to Objects with Masses Below the Limit for Stable Hydrogen Fusion in Stellar Interiors S Ori 70: still a strong cluster planet candidate E.L. Martin (1) (1) Instituto de Astrojis ica de Canarias Avda. Via Lactea, E-38200 La Laguna, Sp ain In this paper I show that the coolest aOrionis cluster planet S Ori 70 is still a strong candidate member despite recent claims by Burgasser et al. that it could be a brown dwarf interloper. The main point of my argument is that the colors of S Ori 70 are significantly different to those of field dwarfs. This object has in fact the reddest H color of all known T dwarfs, - K a clear indication of low gravity according to all published models. In a J - H versus - diagram, S Ori 70 lies in the region where models of ultracool dwarfs predict that low gravityH K objects should be located. I conclude that S Ori 70 is still a strong candidate member of the aOrionis open cluster. I briefly discuss additional observational tests that can be carried out with existing facilities to verify the aOrionis membership of this cluster planet candidate. Keywords: stars: very low mass stars and brown dwarfs. 1 Introduction The term brown dwarf (BD) refers to objects with masses below the limit for stable hydrogen fusion in stellar interiors. For solar composition, this limit was calculated to be 0.08 M0 by Kumar (1963) and Hayashi & Nakano (1963). Modern calculations have changed the value of this boundary only slightly. For example, Baraffe et al. (1998) give 0.072 M0 for solar metallicity. There is no consensus in the community about the minimum mass of brown dwarfs (Boss et al. 2003). Some argue for the deuterium burning limit at 13 Jupiter masses (for solar composition) . Some prefer a limit at around 10 Jupiter masses, where there appears to be a sharp rise in the number of extrasolar planets detected by high-precision radial velocity surveys around main-sequence stars. Finally, it has also been proposed to use a limit at 5 Jupiter masses where the mass-radius relationship of substellar objects changes its sign. In this paper we continue to use the deuterium limit as the massbounda ry between brown dwarfs and planets 81 we do not have any strong reason to change the nomenclature adopted in our previous b,•< dll:iC papers. Another term used in this paper is that of ultracool dwarf, which refers to small objects with very cool effective temperatures. Since 1997, two new ultracool spectral classes have been adopted to extend the classical OBAFGKi\1system into cooler temperatures. The L dwarfs are characterized by weak or absent TiO bands, and very broad NaI and KI lines in the optical spectrum (Martin et al. 1997, 1999; Kirkpatrick et al. 1999, 2000). The T dwarfs are charac­ terized by methane bands in the near-infrared spectra (Oppenheimer et al. 1905; Burgasser et al. 2002; Geballe et al. 2002). Current estimates of the temperatures of ultracool dwarfs range from about 2400 K to 1400 K for L dwarfs, and from 1400 K to 700 for dwarfs (Basri et al. K T 2000; Vrba et al. 2004). Most of the known ultracool dwarfs have been identified in the general field by the wide area surveys 2MASS and SDSS. These objects consist of a mixed population of very low-mass stars, brown dwarfs and free-floating planets formed in different star-formation events during the lifetime of the Milky Way. Their individual ages, chemical compositions and masses are not known. Our best chance to study a population of ultracool dwarfs of known age, chemical composition, and uniform distance is to find them in open clusters where the stellar populations are well characterized. Two of the firstbrown dwarfs identified werelocated in the Pleiades open cluster (PP! 15 and Teide 1, Stauffer et al. 1994; Basri et al. 1996; Rebolo et al. 1995). Now, we know several dozens of bona fide brown dwarfs in the Pleiades and in other open clusters. The uOrionis open cluster has been a region where our group has concentrated many efforts to reveal the substellar population (see Zapatero Osorio et al. 2003 for a recent review). It offers several advantages: (1) It is young (3-8 Myr) and, thus, the substellar objects are relatively bright and hot (Bejar et al. 2001), but not so young that the theoretical models cannot be reliably used to obtain masses (Baraffe et al. 2001). (2) It has very little extinction (Lee 1968), probably because the parental cloud hasbeen blown away by the 0-type star at the center of the cluster. (3) It is relatively nearby (distance 350 pc). (4) It is moderately rich and dense. Bejar et al. (2004, in preparation) estimate a peak central density of 0.2 members per square arcminute. So far the coolest and faintest candidate member that we have found in the uOrionis cluster is the dwarf candidate S Ori 70 (Zapatero Osorio et al. 2002). It was found in a pencil-beam T deep mini-survey of only 55 square arcminutes with a sensitivity of 21 magnitude in the J and fl-bands carried out with the 4.2-meter William Herschel Telescope in the Observatorio de! Roque de los Muchachos. Follow-up near-infrared photometry and low-resolution spectroscopy were obtained with NIRC at the 10-meter Keck I telescope. The photometry is summarized in Table 1, together with the data for field T dwarfs of similar spectral type obtained from the literature. A mid-resolution spectrum in the K-band obtained with NIRSPEC on Keck II was presented in '.VIartfn & Zapatero Osorio (2003). We have claimed that both the NIRC and the NIRSPEC spectra are best fitted with synthetic spectra with low gravity (log g=3.5), which is consistent with membership in the uOrionis open cluster. We have estimated a mass of 3 .Jupiter masses for an age of 3 Myr, making S Ori 70 the least massive object observed directly outside the solar system. 2 A critique of I3urgasser et al. 's paper I3urgasser et al. (2004) have carried out an independent analysis of our Keck NIRC and NIR­ SPEC data of S Ori 70. We sent them our raw and reduced spectra. Burgasser ct al. reprocessed our spectra using their own software and found the same results a5 us for S Ori 70 but not for the comparison object 2MASS J055919-14CM. Apparently, our NIRSPEC data reduction for the 2MASS comparison ohject was incorrect, and it turned out to a field rather than a T dwarf. he star Table 1: Comparison of photometric data of field T4-T8 <lwarfs with S Ori 70 Name SpT J J-H H-I< Ref. 2MASS 2254+3123 T4 15.01±0.03 0.06±0.04 -0.0S±0.04 K04 SDSS 0000+2554 T4.5 14.73±0.05 -0.01±0.06 -O.OS±0.04 K04 SDSS 0207+0000 T4.5 16.63±0.05 -0.03±0.07 0.04±0.07 102 SDSS 0926+5S47 T4.5 15.47±0.03 0.05±0.04 -0.0S±0.04 102 2MASS 0559-1404 T4.5 13.57±0.03 -0.07±0.04 -0.09±0.04 102 SDSS 0742+2055 TS 15.60±0.03 -0.35±0.04 -0.11±0.04 K04 SDSS OS30+012S TS.5 15.99±0.03 -0.lS±0.04 -0.21±0.04 K04 SDSS 0741+2351 TS.5 15.S7±0.03 -0.25±0.06 0.00±0.07 K04 2MASS 2339+ 1352 T5.5 15.Sl±0.03 -0.19±0.04 -0.17±0.04 K04 SOri70 T5.5 20.2S±0.10 -0.14±0.15 0.64±0.25 MZ002 GI 229B T6 14.33±0.05 -0.02±0.07 -0.07±0.07 199 SDSS 1231+0S47 T6 15.14±0.03 -0.26±0.04 -0.06±0.04 K04 SDSS 2124+0100 T6 15.SS±0.03 -0.24±0.04 0.05±0.04 K04 2MASS 0937+2931 T6 14.29±0.03 -0.3S±0.04 -0.72±0.04 K04 2MASS 1225-2739 T6 14.SS±0.03 -0.29±0.04 -0.11±0.04 102 SDSS 1110+0116 T6 16.12±0.05 -0.10±0.07 0.17±0.07 102 2MASS 1047+2124 T6.5 15.46±0.03 -0.37±0.04 -0.37±0.04 K04 SDSS 175S+4633 T7 15.S6±0.03 -0.34±0.04 O.OS±0.04 K04 2MASS 1217-0311 TS 15.56±0.03 -0.42±0.04 0.06±0.04 102 GI 570D TS 15.33±0.05 0.05±0.10 0.01±0.19 BOO 2MASS 0727+ 1710 TS 15.19±0.03 -0.4S±0.04 -0.02±0.04 K04 Burgasser et al. compared the spectra of S Ori 70 with spectra of fieldT brown dwarfs (bdTs) obtained by them, and claimed to find a good match between our object and old brown dwarfs. In their Figure 2, they showed a comparison of their reduction of our NIRSPEC spectrum of S Ori 70 and their NIRSPEC spectrum of the field bdT7 2MASS1553+1532. In their Figure 4, they showed a comparison of our NIRC spectrum of S Ori 70 with their NIRC spectrum of the field bdT6.5 2MASS1047+2124. From these comparisons they claimed that S Ori 70 is a field brown dwarf that coincidentally lies in the line of sight of the cluster. If the interpretation of Burgasser et al. is correct, S Ori 70 should be at a distance of only 75 to 100 pc, instead of 350 pc.
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